CPAP or BiPAP, which mode is best?

This is a quick introduction to the topic of choosing the best Non Invasive Ventilation (NIV) mode to treat a patient in Acute Respiratory Failure (ARF).

Selecting the most apropriate mode of non-invasive ventilation depends on the catagory of respiratory failure.

Acute respiratory failure can be divided into two catagories based on the root cause:

  1. Hypoxaemic
  2. Hypercapnic.

Type 1. Hypoxaemic.

Failure to oxygenate.
Problems with movement of oxygen from the lungs to the blood. Normally associated with issues in the difusion of oxygen from the alveoli to the pulmonary circulation.

Type 2. Hypercapnic.

Failure to ventilate.
Problems with moving gas in and out of the lungs.
Characterised by increased arterial carbon dioxide (CO2) levels.
Usually due to inadequate spontaneous ventilation from:

  • decreased respiratory drive
  • increased work of breathing (WOB).
  • problems with airway conduction.

Some examples include:

t1t2

Once the underlying problem has been identified the best mode of non invasive ventilation can be selected:

CPAP:

Provides one single selected pressure into the patients mask throughout their respiratory cycle.
For example:

On inspiration there will be 10 cmH2O pressure supplied to the patients mask.

As the patient breathes in, this addditional pressure in the mask that is ‘sealed’ to the patients face will assist with inspiration (ie decrease his work of breathing or WOB)

The increased pressure in the mask will flow all the way down the airway to the alveoli. This will fill alveoli that may otherwise be partially collapsed (recruitment), increasing the surface area that is available for gas exchange.

On expiration there will still be 10 cmH2O pressure supplied to the patients mask.

The patient is now breathing out against the resistance of this pressure. The positive pressure will now ‘hold’ those alveoli open at the end of expiration increasing the time available for gas exchange (known as: increased functional residual capacity).

This positive pressure is also known as Positive End Expiratory Pressure or PEEP.
So, CPAP = PEEP.

Bi-Level ventilation (or BiPAP)

Two separtate pressures are selected.
One for inspirataion (IPAP).
One for expiration (EPAP).

This provides the benefits of CPAP with the additional benefit of an increased support during inspiration. This extra support above the setting for CPAP is called: Pressure Support.
For example, if the doctor was to select IPAP=15 and EPAP=10:

On inspiration there will be 15 cmH2O pressure supplied to the patient mask.

This higher pressure will provide an incresed support as the patient breathes in. Further decreasing WOB (and therefore resp muscle fatigue and myocardial oxygen demand).
This incresed pressure also encourages the patient to take bigger breaths (or Tidal Volumes) which helps remove any excess CO2 from the bloodstream.

On exhalation there will be a lesser pressure of 10 cmH2O supplied to the patients mask.

So it will be a little easier to breathe out for the patient. But there will still be a resistance and a pressure at the end of exhalation with the same effects as those of CPAP.

Pressure Support: As the inspiratory (IPAP) pressure is 5 cmH2O above the expiratory (EPAP) pressure the pressure support is said to be 5 cmH2O.
So, we have : IPAP – EPAP = Pressure Support (PS)

Increasing pressure support by widening the distance between IPAP and EPAP will decrease the patients WOB.

Increasing EPAP ( which is the same thing as PEEP) will assist to increases the patients arterial oxygen concentration.

So in summary, selection of the best mode of NIV is dependant on the type of respiratory failure experienced by the patient:

niv


NOTE: This article is not intended to be a comprehensive teaching of the principles of Non-Invasive ventilation, CPAP or BiPAP. It is a quick explanation aimed at those having diffiuclty with first principles of this topic. For more detailed information see: http://lifeinthefastlane.com/ccc/non-invasive-ventilation-niv/

7 thoughts on “CPAP or BiPAP, which mode is best?

  1. I’ve never heard that BIPAP is a mandatory mode. It’s always been a face-mask (therefore spontaneously-triggered) measure where I have been involved. In intubated, ventilated patients it is then referred to as CPAP + PS.
    I think we should just assume that this article is referring to non-ventilated patients and avoid complication and confusion.

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  2. It’s a very nice summary but misses the crux of the matter. BiPAP is a mandatory mode. If a patient requires mandatory ventilation they should be intubated. CPAP with Pressure Support (CPAP/PS) is a spontaneous mode with additional support, as you described. The difference between BiPAP and CPAP/PS is one is time triggered (mandatory) and the other patient triggered (spontaneous). EPAP = CPAP/PEEP but IPAP doesn’t = PS, rather it is pHIGH. I’d recommend adjusing the article appropriately.

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    1. I’d have to disagree with your use of the NIV terminology here James. The term BIPAP is used interchangeably in the U.K. The term BIPAP in the context of NIV mode that utilises a IPAP and EPAP pressure which cycles based on inspiratory and expiratory flow triggered by the patient. The differences between between pressure support via an ETT or Tracey and NIV EPAP and IPAP are pure somantics and the differences are largely irrelevant and only confuses people new to invasive or non invasive ventilation. It’s a little like looking at differences between BIPAP/BI-Level/DUOPAP using an invasive ventilator, they are all the same thing, any difference again is somantics which just confuse people. BIPAP/BI-Level/DUOPAP In the context of invasive ventilation has manditory aspects if the patient is apneic. However, many ITU NIV machines (respironics V60) actual have duel modes of BIPAP that can be spontaneous or duel modes. However, much of this is beyond the basic concepts that this blog is trying to impart to the novice

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